In a conventional gas turbine engine, the engine is controlled by monitoring the speed of the engine and adding a proper amount of fuel to control the engine speed. Specifically, should the engine speed decrease, fuel flow is increased thus causing the engine speed to increase. Similarly, should the engine speed increase, fuel flow is decreased causing the engine speed to decrease. In this case, the engine speed is the control variable or process variable monitored for control.
A similar engine control strategy is used when the gas turbine is connected to an AC electrical grid in which the engine speed is held constant as a result of the coupling of the generator to the grid frequency. In such a case, the total fuel flow to the engine may be controlled to provide a given power output level or to run to maximum power with such control based on controlling exhaust gas temperature or turbine inlet temperature. Again, as the control variable rises above a set point, the fuel is decreased. Alternatively, as the control variable drops below the set point, the fuel flow is increased. This control strategy is essentially a feedback control strategy with the fuel control valve varied based on the value of a control or process variable compared to a set point.
In a typical combustion system using a diffusion flame burner or a simple lean premixed burner, the combustor has only one fuel injector. In such systems, a single valve is typically used to control the fuel flow to the engine. In more recent lean premix systems however, there may be two or more fuel flows to different parts of the combustor, with such a system thus having two or more control valves. In such systems, closed loop control is based on controlling the total fuel flow based on the required power output of the gas turbine while fixed (pre-calculated) percentages of flow are diverted to the various parts of the combustor. The total fuel flow will change over time.
In addition, the desired fuel split percentages between the various fuel pathways (leading to various parts of the combustor) may either be a function of certain input variables or they may be based on calculation algorithms using process inputs such as temperatures, air flow, pressures etc. Such control systems offer ease of control due primarily to the very wide operating ranges of these conventional combustors and the ability of the turbine to withstand short spikes of high temperature without damage to various turbine components. Moreover, the fuel/air ratio fed to these combustors may advantageously vary over a wide range with the combustor remaining operational. A wide variety of such control strategies can be employed and a number of these have been described in the literature.
A properly operated catalytic combustion system can provide significantly reduced emissions levels, particularly of NOx. Unfortunately, however, such systems may have a much more limited window of operation compared to conventional diffusion flame or lean premix combustors. For example, fuel/air ratios above a certain limit may cause the catalyst to overheat and lose activity in a very short time. In addition, the inlet temperature may have to be adjusted as the engine load is changed or as ambient temperature or other operating conditions change.